1909] on Experiments at High Temperatures and Pressures. 553 



opaque : the external form of the crystals is still monoclinic, but they 

 are merely pseudomorphs of the original crystals. To obtain large 

 octahedral crystals we may suppose that we begin by melting sulphur 

 and raising the temperature and pressure till the former stands at 

 160° C. or over, and the latter at not less than 1600 kg./cm.^ (10-16 

 tons/sq. inch). 



If we then slightly reduce the temperature or raise the pressure, 

 we shall have the crystallisation of the sulphur in the rhombic form. 

 By maintaining the pressure as the mass cools and when it is cold 

 releasing the pressure, we should finally extract rhombic crystals. To 

 this we may of course add that we need not expect crystals of any size 

 unless we cool at the proper rate. It appears that there are at least 

 two phenomena requiring attention in relation to the production of 

 crystals — one is the relation between the amount of undercooling 

 necessary to induce spontaneous crystallisation, and the other is the 

 rate at which the crystals will grow when they have once started. 

 If we want large crystals, we must not have an excessive number of 

 points of spontaneous crystallisation ; nor must we have too high a 

 rate of crystal growth, or the crystals will by all experience tend to 

 be felted together. The temperature condition giving birth to the 

 most favourable number of spontaneous centres is not necessarily the 

 temperature at which crystals grow to the largest size, so there is really 

 no escape from finding by direct trial the most effective way to go to 

 work. 



Another possibility is brought to light by an examination of a case 

 of pseudo-equilibrium such as that of phenol. Here we have three 

 regions — in one No. 1 alone is stable, in another No. 2, and in the third 

 both Nos. 1 and 2 are stable. The case of iodide of silver is similar, 

 but more complicated. If in the area C we change the pressure, the 

 temperature remaining constant and the material consisting of a mix- 

 ture of the two stable phases, we can alter the proportions in which 

 these phases exist,- but we cannot cause either of them to disappear. 



A notable case of this kind is that of graphite and diamond, both 

 perfectly stable in presence of each other at atmospheric pressure up 

 to a temperature nearly that of the electric arc, say about 3000° C. 

 If there be any similarity between the carbon and phenol diagrams, 

 diamond would correspond to variety No. 2 of phenol, and graphite to 

 variety No. 1, heat being evolved in both cases when the less dense 

 modification changes into the denser. If we desire to obtain phenol 2 

 from phenol 1, we note that down to a temperature of - 20° C, we 

 should require to keep the pressure always above about 600 kg. /cm.- ; 

 otherwise the operations would be similar to those described in the 

 case of rhombic sulphur. 



Similarly, to convert graphite to diamond on this analogy we 

 should have to raise the temperature and pressure together to some 

 unknown values and then let the product cool — keeping up the 

 pressure meanwhile. 



2 2 



